As improved techniques for obtaining artificial leather which is possessed of handling or feeling and quality of the naturally occurring leather having high quality, there have been proposed such techniques as those relating to very fine fibers serving as fibrous base materials; those relating to polymers with which fibrous base materials are coated or impregnated; and those concerning pre-treatments of fibrous base materials prior to the coating or impregnation thereof with the polymers. And means for diluting the polymers used in these techniques are those which make use of, for instance, dimethylformamide (DMF), i.e., an organic solvent harmful to the human body.
In addition to such a problem that this organic solvent would adversely affect the working environment, for instance, in processing fields, the organic solvent suffers from a problem in that since it requires the use of a large amount of water for hot water-washing or water-washing after the treatments, this would result in water pollution and/or air pollution if the used water is discarded. Therefore, the organic solvent present in the discharged water or the exhaust gas must be recovered and the solvent thus recovered should be post-treated in order to eliminate the foregoing problems. Thus, the use of such an organic solvent suffers from such a problem that it requires a great deal of labor and much expenses for these treatments.
In addition, the techniques which make use of organic solvents suffer from a further problem in that they require a large number of processing steps since they comprise, as shown in FIG. 7, the steps of shrinking a fibrous base material; impregnating the material with polyvinyl alcohol (PVA) (i.e., a step of treating the fibrous base material with polyvinyl alcohol to inhibit any adhesion of a polymer to the fibers constituting the base material); drying the impregnated base material; diluting a polymer; coating or impregnating the dried base material with the polymer; solidifying the polymer; washing the base material with hot water or water; squeezing the base material; drying it; and rolling or winding the base material to obtain a product.
To solve these problems, it would be conceivable to inhibit the use of any organic solvent, i.e., to adopt a method comprising the steps of diluting, with water, a polymer in the form of an aqueous emulsion such as a polyurethane resin stock solution in the form of an aqueous emulsion to an appropriate concentration, impregnating a fibrous base material such as a nonwoven fabric with the resulting dilute solution, and then fixing the polyurethane resin onto or into the fibrous base material through drying and curing like the conventional techniques which employ organic solvents.
This method does not use any organic solvent and therefore, permits the elimination of the PVA-impregnation step and the steps subsequent thereto as well as the hot water-washing or water-washing step and the steps subsequent thereto, which are common in the methods utilizing organic solvents. For this reason, the method permits the preparation of a desired product through the steps of shrinking a fibrous base material, coating the base material with a polymer in the form of an emulsion, pre-drying the coated base material, drying it, and adhering or fixing the polymer thus solidified to the base material. Thus, the method is advantageous in that the number of steps required can be reduced, to some extent, as compared with that required for the method which make use of organic solvents.
However, it has been found, as the results of the supplementary examination of this method, that the method suffers from the following problems:
One of these problems is to cause, at the initial stage of drying, the so-called migration phenomenon wherein the polyurethane resin solid content in a polyurethane resin liquid in the form of an aqueous emulsion, with which the fibrous base material is impregnated, travels upon evaporation of water contained in the emulsified aqueous polyurethane resin liquid which is impregnated into the base material. This phenomenon in turn leads to the movement of the resin component from the interior of the base material to the front and back faces thereof along with the evaporated water component and the reduction of the polyurethane resin content in the interior of the impregnated base material. This becomes a main cause of impairing the handling or feeling of the resulting product.
In addition, if the base material impregnated as mentioned above is dried by dry heating and then the polymer is solidified and adhered or fixed to the base material through dry heating using of hot air (of 120 to 150.degree. C.) and if a polyurethane resin in the form of an aqueous emulsion is, for instance, used as the impregnation liquid, the rate of the polyurethane resin solidified and adhered or fixed to the fibers in the fibrous base material is low and on the order of not more than 10%, while the rate observed when using an organic solvent ranges from 25 to 50%. Therefore, there has not been obtained any product having satisfactory handling or feeling.
Due to the increased content of polymer adhered or fixed to the fibrous base material observed when using the conventional method which comprises the steps of drying through dry heating and then solidifying and adhering or fixing through dry heating, the polyurethane resin component is adhered to the fibers in the base material and hardened therein as will be seen from the electron micrograph 5, as a substitute for a figure, showing the structure of artificial leather and therefore, the method suffers from a problem in that it is difficult to process the product in the subsequent processing steps such as the processing step when making use of a technique for processing island-type (or sea island-type) fibers into microfibers, or in the subsequent dyeing step.
In other words, the artificial leather processed according to this method is then subjected to a leaching processing step and/or a dyeing processing step in a jet dyeing machine or a pad-steam dyeing machine. However, the artificial leather of this type causes, for instance, insufficient operation of the jet dyeing machine, the formation of specks due to the leaching in the leaching step and the formation of specks or spots due to dyeing in the dyeing step. In the pad-steam dyeing step, there are also observed such disadvantages that the sheets of artificial leather are liable to come in contact with each other in the steam and that the contact therebetween becomes a cause of color stain.
It is thus an object of the present invention to obtain artificial leather which is soft and has quality almost comparable to that of the naturally-occurring leather, while using a polyurethane resin stock solution in the form of an aqueous emulsion which is never accompanied by problems such as environmental pollution.